This invention relates to a method for producing a semi-conductor body which is useful in a jet engine igniter of the high energy type, which, in service, is fired by an ignition system which includes a capacitor. The semi-conductor body is incorporated in the high energy igniter so that a portion of a surface thereof is adjacent a spark gap between a center electrode and a ground electrode. The function of the semi-conductor is to enable the spark discharge across the igniter gap to occur at lower applied voltages. Lower voltages decrease the weight and bulk of insulation required at high altitudes and thereby permit higher energy storage and discharge.
Various electrically semi-conducting ceramic bodies have heretofore been suggested and used in igniters for low voltage ignition systems: see, for example, U.S. Pat. Nos. 3,037,140 and 3,046,434. Improved semi-conductor bodies made by hot pressing mixtures of alumina and silicon carbide are disclosed in U.S. Pat. No. 3,558,959. The hot pressed alumina silicon carbide semi-conductors perform satisfactorily under operating conditions more severe than those previously known semi-conductors were capable of withstanding, but their production proved to be complicated and expensive. For example, it was not found to be possible to produce bodies of the required shape by the hot pressing technique; instead, it was found to be necessary to shape the extremely hard alumina silicon carbide body subsequent to hot pressing by boring, honing and grinding with diamond tools.
In U.S. patent application Ser. No. 321,563 (Series of 1970) it was disclosed that a very satisfactory semi-conductor body can be produced without hot pressing or extensive shaping operations. The method disclosed and claimed therein essentially the steps of pressing a batch of silicon carbide, alumina, and various calcium and magnesium compounds into a shape; firing the shape first in air; and firing the resulting body a second time in an inert atmosphere, producing a desired semi-conductor body. The first firing is at a temperature of from about 2050.degree. to 2100.degree.F. for a time sufficiently long to convert in situ a predetermined portion of the silicon carbide to silicon dioxide. The resulting body is then fired in an inert gas atmosphere to a temperature from about 2450.degree. to 2750.degree.F. to produce a semi-conductor body having a specified apparent porosity.
It has also been suggested (see, for example, U.S. Pat. Nos. 3,376,367 and 3,573,231) that semi-conductors can be produced from silicon carbide and aluminum silicate or the like by forming an article of the desired shape, embedding the article in a mass of silicon carbide particles, and firing. The aluminum silicate can be a part of the batch from which the original shape is formed, or it can be produced in situ by firing the shape in air for one-half hour at 2000.degree.F. to cause oxidation of silicon carbide to SiO.sub.2, which can then react with alumina in the shape to produce the aluminum silicate.
The mechanism by which an Al.sub.2 O.sub.3 - SiC semi-conducting body operates is not fully understood, but two theories have been proposed which in no way should be construed as a limitation to the scope of this invention.
One theory suggests that when a voltage is applied to the center electrode, there is a limited flow of current along the semi-conductor surface. This current flow causes ionization of gas in the spark gap. The ionization enables a spark discharge to occur at a lower voltage than would be required without the ionization. Discharge of the previously charged capacitor occurs when there is a spark between the ground and center electrodes. The large size of the capacitor is responsible for the high energy nature of the spark.
Another theory suggests that because a small space of about 0.0005 inch exists between the center electrode and the semiconducting body, electrical charges of opposite polarity build up on the surface of the center electrode and the semiconductor such as in the polarization of opposing faces of a capacitor. Ionization of gas in this small space or microgap within the ignitor gap permits an initial spark discharge at a low applied voltage. This partial discharge is believed to cause a cascade ionization and discharge across the main gap.
An extension of this theory proposes that the porosity of the semi-conductor surface assists the cascade process by providing a series of microgaps between conducting silicon carbide grains which may become charged, ionized and discharged in rapid succession. The presence of a nonconducting phase such as alumina serves not only to bond the conducting grains of silicon carbide, but also to prevent a direct short circuit. Controlled spacing and contact of the silicon carbide grains is obtained by means of the porosity and the alumina as well as the grain size and amount of silicon carbide.
Optimum control of semi-conductor performance has been found to occur when the porosity is between 10 and 25% and when surface electrical resistance is between 1 and 200 megohms at 500VDC.
The porosity has also been found to have a desirable influence on the resistance to spark erosion of the semi-conducting body. The low thermal conductivity imparted by the porosity is believed to restrict the high heat generated by the spark to the surface. This would limit the destructive melting and expansion stresses to the surface.
It is apparent that a very strong bond is necessary to hold the silicon carbide grains in the presence of the large required void space. Aluminum oxide has a high mechanical strength relative to silicates, glass and many other oxides. The mechanical integrity of the network of alumina which forms after sintering is far superior to other sintered bond materials. Aluminum oxide has previously given too low a density when it is the sole bond material in cold pressed and sintered bodies. It could only be satisfactorily used when formed by hot pressing or when fired in an embedment of grain in air in prior art to form an aluminum silicate with the oxidation product of silicon carbide.